The invention relates to apparatus for manipulating substrates and, more particularly, to an apparatus for lifting a substrate from the surface of a pedestal in a semiconductor substrate processing system.
A conventional semiconductor wafer processing system contains a reaction chamber within which a wafer is supported by a pedestal subsystem having a pedestal that cooperates with a lift pin assembly. A wafer transport robot operating in cooperation with a lift pin assembly positions the wafer above the pedestal. The robot moves the wafer into the chamber through a slit valve. The lift pins of the lift pin assembly extend above the surface of the pedestal and lift the wafer from the robot arm. The lift pins are usually elevated, by a lift mechanism, to provide clearance for the robot blade of the robot arm. The lift mechanism typically includes a lift plate in contact with the bottom ends of the lift pins and being driven by an actuator to move the lift pins up and down. The lift mechanism, usually under control of a computer control system, then lowers the lift pins below the pedestal so that the wafer is placed onto the surface of the pedestal. The pedestal may either mechanically or electrostatically clamp, i.e., chuck, the wafer to the pedestal.
After the wafer is placed onto the support surface of the pedestal, the lift pins continue to descend into the pedestal to a fully retracted position. Then, the wafer is usually chucked and one or more semiconductor fabrication process steps are performed in the chamber, such as deposition or etching films on the wafer. After completion of the process steps, the lift mechanism extends the lift pins to raise the wafer above the pedestal so that the wafer can be removed from the chamber via the robotic transport. When using an electrostatic chuck, before the lift pins can raise the wafer, the wafer must be electrically dechucked, i.e., the electrostatic force retaining the wafer on the pedestal must be removed or canceled. However, even after applying a conventional dechucking method, a residual charge still remains on the wafer and pedestal due to charge migration and/or field emission charging. As such and without damaging the wafer, the lift pins must forcibly lift the wafer to separate the wafer from the pedestal.
Additionally, different wafer sizes, e.g., 200 mm or 300 mm, are being used in semiconductor processing. Longer lift pins may be needed for processing some wafers to adequately provide clearance for robotic transport. Increasing the length of the lift pins increases the tendency for the lift pins to bow or slide relative to the lift plate, especially with the additional lifting and clamping forces involved with larger wafers. FIG. 1 shows sliding of the lift pins 10 on the lift plate 12 as the lift plate 12 moves the lift pins 10 and the substrate 14 up and down with respect to the substrate holder or pedestal 16. When sliding of the lift pins 10 occurs, the substrate 14 may not be properly aligned with the substrate support 16 when the lift pins 10 are lowered to place the substrate 14 on the support 16. For instance, the support surface 18 of the substrate support 16 may have a pocket to receive the substrate 14. If the substrate 14 is out of alignment with respect to the support surface 18, the substrate 14 may be tilted. This may affect the spacing between the substrate and the process gas flow (e.g., from a faceplate disposed above the substrate) or the heating of the substrate 14 by a heater provided in the substrate support 16.